1. Technical Field
The present invention is directed to improving immunity to channel distortions of multiplexed analog component television signals by independently transmitting a clock recovery burst and a DC restoration reference level during the horizontal blanking interval of a video line. More particularly, the clock recovery burst and the DC restoration reference level are transmitted on independent lines.
2. Background Information
Time division multiplexed analog component (MAC) television signals, a typical line of which is shown with reference of FIG. 1, includes a horizontal blanking interval (HBI) 12, in which no picture information is transmitted, followed by a chrominance signal 14 and a luminance signal 16, either of which may be time-compressed. Between the chrominance and luminance signals is a guard band 18 to assist in preventing interference between the two signals.
The MAC color television signal of FIG. 1 is obtained by generating conventional luminance and chrominance signals (as would be done to obtain a conventional NTSC or other composite color television signals) and then sampling and storing them separately. Luminance is sampled at a luminance sampling frequency and stored in a luminance store, while chrominance is sampled at a chrominance sampling frequency and stored in a chrominance store. The luminance or chrominance samples may then be compressed in time by writing them into a store at their individual sampling frequency and reading them from the store at a higher frequency. A multiplexer selects either the luminance store or the chrominance store, at the appropriate time during the active line period, for reading, thus creating the MAC signal. If desired, audio samples may be transmitted during the HBI; these are multiplexed (and may be compressed) in the same manner as the video samples. The sample rate at which all samples occur in the multiplexed MAC signal is called the MAC sampling frequency.
In the transmission of all typical MAC signals, a reference clock frequency burst is also transmitted during the HBI. The reference burst is typically about ten cycles of a constant amplitude sinusoid at a subharmonic of the MAC sampling frequency and is used at the receiver for both clock recovery and DC restoration (clamping). The frequency of the reference clock burst is used for clock recovery, while the average of the reference clock burst (ideally zero) is used for clamping. It is of great importance to accurately DC restore the video signal after transmission and clamp on the luminance (or pedestal) value upon which these reference clock burst sinusoids are superimposed. The chrominance values obtained for transmission are relative to a given reference brightness level, represented by the DC level of the signal. Accordingly, it is necessary that the received signal be referred to that reference level to provide accurate reconstruction of the received signal on a display. This reference level is hereinafter referred to as the DC restoration reference level.
Several problems exist with the prior art method of obtaining the DC restoration value by averaging the reference clock burst. For example, if the discriminator at the receiver (Foster-Seeley or FM discriminator) is misaligned, the average value of the reference clock burst is not at its ideal zero reference point. Additionally, if a large data value either before or after the reference clock burst occurs, when the clock burst is low pass filtered, the data value may cause interference and consequently shift the DC level. Furthermore, any other distortion classically inherent in FM discriminators will cause an error to occur when the DC restoration reference level is obtained by averaging the reference clock burst.
This error, sometimes referred to as chrominance/luminance intermodulation, is a non-linear distortion particularly likely to be encountered in FM discriminators as a result of misalignment or drift. It may also occur in a wide variety of baseband video amplifiers, especially as a result of differences in circuit component value tolerences. A test signal has been developed specifically to measure this distortion in an FM demodulator. See Int'l Radio Consultative Comm., Recommendations And Reports Of the CCIR, Transmission of Sound Broadcasting and Television Signals Over Long Distances, Vol. XII, pps. 13, 20 (XVth Plenary Assembly, Geneva 1982).
In FM systems operating at or below the threshold C/N, the clock recovery burst may be corrupted by impulsive noise. This is because, during transmission of the bursts, the carrier frequency deviates substantially away from the center of the channel. Such impulsive noise can therefore corrupt any DC restoration process based upon the prior art method of clamping during the burst.
One solution to this problem is to provide a separate clock recovery burst and DC restoration reference level in each transmitted line. If the reference clock burst and the DC restoration reference level were both about 3 us per line, these two intervals would represent approximately 10% of the typical 63.5 us line. This approach is uneconomical; not only will the bandwidth required for transmission be increased, but the signal-to-noise ratio would also be increased, requiring a larger antenna reflector and/or a more costly Low Noise Amplifier.